Method for producing rod-shaped silicon monocrystals with homogeneous antimony doping over the entire rod length



. March 17, 1.970 R.'KAPPELMEYER ETAL 3,501,405

METHOD FOR PRODUCING ROD-SHAPED SILICON MONOCRYSTALS WITH HOMOGENEOUSANTIMONY DOPING OVER .THE ENTIRE ROD LENGTH Filed June 8, 1967 UnitedStates Patent US. Cl. 25262.3 9 Claims ABSTRACT OF THE DISCLOSUREDescribed is a method of producing rod-shaped silicon monocrystalshaving homogeneous antimony doping over the entire rod length, bypulling from a melt. The monocrystal is pulled by a crystal seed from amelt contained in a crucible, with an appropriately selected antimonycontent, whereby a portion of the antimony present in the melt isvaporized during the growth of the crystal. The pulling process iscarried out in an evacuable reaction vessel, in a gaseous atmosphere andat a reduced pressure.

The present invention relates to a method of producing rod-shapedsilicon monocrystals with homogeneous antimony doping over the entirerod length by pulling from a melt.

Several fields of the semiconductor art require silicon rods having ahomogeneous antimony doping across their entire length. These rods maybe divided by means of an appropriate dividing process, such as sawingor breaking, into disc-shaped bodies and used as carrier crystals(substrate discs) for epitactic growth layers. The latter may then beprocessed into semiconductor components, without the need for additionalor dividing processes. It is preferred, particularly for seriesproduction, that all discs have the same doping concentration, as thisis a prerequisite for the reproducible manufacture of a plurality ofsemiconductor components having equal or almost equal characteristics.However, the production of rod-shaped silicon monocrystals havinghomogeneous antimony doping over their entire length, entailsconsiderable difliculties in the methods heretofore used. Above all, thespecific resistance of a rod length, corresponding approximately to 90%of the crystallized melt, diminishes to about 40% of the value, at thebeginning of the rod, in the methods which employ normal conditions(about 760 mm. Hg). Monocrystalline rods having such steep dopinggradients are largely unsuitable for direct subsequent processing intocarrier crystals.

The present invention utilizes a method whereby the monocrystal ispulled by a crystal seed from a melt held in a crucible containing anappropriate amount of antimony. During the growth of the crystal, aportion of the antimony contained in the melt is vapor-deposited at suchamounts that the increase in antimony concentration caused by thedistribution coefficient is compensated for, by means of vaporization ofthe antimony.

The present method is characterized by the fact that the pulling processis carried out in an evacuable reaction vessel, under a protectiveatmosphere and at a decreased pressure, whereby the atmosphere ispreferably comprised of argon. To this end, an argon pressure of abouttorr (10 mm. Hg) is adjusted in the reaction vessel, after the crystalseed has been dipped into the melt. This pressure, during the furthercourse of the 3,501,406 Patented Mar. 17, 1970 pulling process, mayeither be maintained at the same value, or may be lowered to a pressureof about 3 torr.

In the present method, silicon is preferably first melted in a highvacuum, at pressures of less than 10 torr, and is thereby freed fromvolatile contaminations.

The silicon monocrystals, produced in accordance with the presentinvention, are particularly suitable for the production of carriercrystals for epitactic growth layers. Directly following the coatingprocess, they may be processed into semiconductor components such astransistors, rectifiers and the like, or they may be processed directlyinto such semiconductor components as transistors, diodes or solid stateintegrated circuits.

Additional details of the present invention will be derived from theembodiment examples described with respect to the figure.

In a reaction vessel 10 a crystal seed 1 is inserted into a holder 2which is connected to a driving device, not shown. The connectionbetween the holder 2 and the driving device is through an intermediarymember 3. By means of this driving device, the crystal seed 1 togetherwith the monocrystalline silicon rod 4, growing thereon, may be rotatedaround its longitudinal axis and may be pulled, in accordance with thecrystal growth, upward from the melt 5 which is contained in quartzvessel 6. The quartz vessel 6 is arranged within graphite vessel 7,which is heated by high-frequency coil 8, positioned outside thereaction vessel 10. The heating action of the highfrequency coil 8 isamplified by the energy concentrator 9. The quartz vessel is heated by aheat transfer from the graphite vessel 7. The temperature of the melt isdetermined by means of a Pt/Pt-Rh thermoelement 11, which is containedin a protective tube 12 of aluminum oxide or quartz. The thermoelement11 may be connected to a regulating control circuit, not shown in thefigure, to control the energy supply and thereby the meltingtemperature. The lower seal of the reaction vessel 10 is the bottomplate 13 through which the tubular vessel holder 14 and the rod-shapedholder 15 for the energy concentrator 9 are hermetically led.Furthermore, an inlet nozzle 16 has been provided through which theprotective gas, e.g. argon, from a storage vessel 17 is introduced tothe reaction vessel 10, via dosing valve 18. A head portion 20 equippedwith a cooling jacket is provided as the upper seal for the reactionvessel 10. The inlet and outlet for the cooling water are throughopenings 21 and 22. The rod holder 2 which is coupled with theconnecting piece 3 is hermetically led through the head 20. Seals 23 and24 are further provided for sealing the reaction vessel. Theunder-pressure in the reaction vessel is produced by the pumpingaggregate consisting of the diffusion pump 25 and the circulating pump26. The block valve 27 is also installed into the pump line. Thepressure is measured by manometer 28 and Penning measuring tube 29.

Two modes of operation are particularly suitable for carrying out themethod of the present invention.

In the first mode of operation, the silicon is first melted at a reducedpressure, for example at 10- torr. The melting temperature is aboutl400-l450 C. The temperature of the melt is then reduced to the pointthat the melt remains just about liquid. Thereafter, argon i introducedfrom the storage vessel into the reaction vessel and the gas pressure isadjusted in the vessel to approximately 500760 torr. After the crystalseed is dipped in and melted on, the pulling of the crystal isinitiated. The antimony, serving as the doping material, is thrown insmall pieces, e.g. balls of equal weight, into the silicon melt, priorto or following the dipping in of the crystal seed. The crystal which isrotated around its longitudinal axis at a speed of about 10-100 r.p.m.,preferably about 50 r.p.m., is now pulled from the antimony doped melt.The pulling speed is approximately 13 mm./min. Thereafter, the gaspressure in the reaction vessel is adjusted to a value of about torr.This value is either maintained or reduced to approximately 3 torr. Thepumping capacity is preferably so adjusted that the flow rate of the gasis at least 3 l./ min.

The amount of material to be used depends on the required dopingconcentration and may be calculated without difficulty from the knowndistribution coefficients of the employed materials as well as from thevaporization rate of the doping material.

In the second mode of operation, a pre-alloying of antimony and siliconis effected. This is then melted at a pressure of about 760 torr. Thismay be done by putting pure antimony, together with the silicon, intothe crucible and melting it. Subsequently, the crystal seed is dippedinto said melt and melted on. The crystal pulling is effected in themanner described in the previous example. Temperature and pressure aswell as the travelling speed of the gas are adjusted analogously.

We have found it to be particularly advantageous to adjust an argonpressure of about 10 to approximately 3 torr, since this pressure notonly effects the desired vaporization rate of the antimony but alsocorresponds to the vapor pressure of the silicon monoxide, at a meltingtemperature of the silicon.

Experience has shown that at a pressure of about 10 torr and less, thesilicon monoxide vaporizes at a high rate of vaporization andprecipitates at the more distant walls of the reaction vessel or atanother location, while at'a normal pressure (even in an argonatmosphere) the silicon monoxide condenses at the upper rim of thequartz crucible. This rapidly growing precipitation is spongy andcrumbles off the crucibles edge. The silicon monoxide which, therefore,falls into the melt leads to considerable disturbances. These may beavoided by following the present invention and by effecting the pullingprocess at a pressure of about 10 torr which may be reduced to 3 torr.

The reduction of the specific resistance across the rod length isvirtually eliminated due to the present invention, as compared to a dropof approximately 60%, occurring in the known methods.

We claim:

1. A method of producing rod-shaped silicon monocrystals havinghomogeneous antimony doping over the entire rod length, by pulling froma melt, which comprises pulling a monocrystal by means of a crystal seedfrom a melt with an appropriately selected antimony content, containedin a crucible, whereby a portion of the antimony present in the melt isvaporized during the growth of the crystal, said pulling process beingcarried out in an evacuable reaction vessel in which is a gaseousatmosphere at a pressure under about 10 torr.

2. The method of claim 1, wherein a pressure of approximately 10 torr ismaintained constant, during the entire pulling process.

3. The method of claim 1, wherein the pressure during the pullingprocess is reduced from about 10 torr to about 3 torr.

4. The method of claim 1, wherein the silicon is melted under a highvacuum at pressures of less than 10* torr.

5. The method of claim 4, wherein the pulling process is carried out inan argon atmosphere.v

6. The method of claim 5, wherein the vessel has a volume of about literand the argon flow rate is about 3-5 l./min.

7. The method of claim 6, wherein the pulling process is carried out ata temperature corresponding approximately to the melting temperature ofthe silicon.

8. The method of claim 7, wherein the silicon monocrystal is pulled fromthe melt at a velocity of about 1 to 3 mm./ min.

9. The method of claim 8, wherein the speed of rotation of the siliconmonocrystal, rotating around its longitudinal axis, is adjusted toapproximately 10 to 100 r.p.m.

References Cited UNITED STATES PATENTS 2,981,687 4/1961 Parmee 25262.33,167,512 l/l965 Ziegler 25262.3 3,296,036 1/1967 Keller 25262.3 X

HELEN M. MCCARTHY, Primary Examiner J. COOPER, Assistant Examiner US.Cl. X.R.

2330l; 148l7l,

